1. Field of the Invention
The present invention relates to electrical fuses. More specifically, the present invention relates to a method and an apparatus for protecting an electrical circuit against excessive currents by a fuse assembly configured to interrupt the flow of current through the electrical circuit by increasing dielectric separation between two ends of a fuse element.
2. Description of the Related Art
A fuse is a safety device that typically protects electrical circuits from the effects of excessive currents, e.g., during an overload condition. The electrical circuit may be overloaded due to excessive current caused by abnormal operation of the electrical circuit, abnormal changes in the load and/or abnormal changes in the electrical circuit's inputs. Most electrical devices such as computers, telecommunications equipment, amplifiers, TV's, and products with embedded electrical systems such as automobiles, aircraft, heating and cooling systems and even space vehicles typically include a protective device, e.g., a fuse.
Printed circuit boards (“PCB”), or the like, on which electrical and/or electronic components are mounted to form electrical circuits are well known in the art. Conventional printed circuit boards typically include through-hole and/or surface mounted components. The surface mounted components are typically mounted on PCB's using surface mounted technology (“SMT”). The fuse is an example of a component included in a typical printed circuit board. In the quest to build printed circuit board assemblies with improved circuit protection that is smaller, faster, and safer, fuse developers are extending their expertise in optimizing the fuse design by improving the operating characteristics and by reducing the footprint.
A fuse assembly typically includes a current-conducting fuse element, e.g., a strip or wire of easily fusible conducting material capable of heating and melting, a dielectric material enclosing the fuse element, and a pair of conducting terminals connected to the fuse element. As is well known, the dielectric material does not readily conduct electricity. Whenever the circuit protected by the fuse is made to carry a current larger than that for which it is intended, the fuse element typically generates heat due to the excessive current flowing through the element, gets heated to its melting point and eventually melts. The melting of the fuse element causes the element to be split transversely into at least two smaller elements separated by a gap. The separation of the element into two elements and a gap, due to melting, has the effect of interrupting the flow of current through the circuit. Depending on the voltage potential across the gap, electrical breakdown of the poor dielectric inside the fuse such as air, or arcing, may occur between the two smaller, separated elements.
Fuses may be packaged differently depending on the application. For example, a screw-bulb-type fuse, commonly used in earlier domestic electrical systems, contains a short bit of wire (the fusible element) enclosed in a dielectric container, e.g., glass, which has a screw-threaded base. The wire is connected to metal terminals at both the screw base and at the side, and the fusible element is viewable for seeing whether the fuse element has melted. The cartridge-type fuse, a type of fuse widely used in industry where high currents are involved, has a fusible element connected between conducting metal terminals at either end of a cylindrical insulating tube, which is typically made from glass or ceramic.
Traditional printed circuit boards have used the cartridge type fuse. The TeleLink® SM fuse manufactured by Teccor Electronics, Irving, Tex., USA, is an example of a cartridge-type fuse used in printed circuit boards with surface mounted components.
A problem with traditional fuses is heat generation caused by arcing across the gap due to high interrupting voltage. The voltage potential between the two remaining pieces of the fuse element may be sufficient to overcome the insulation provided by the air or other substance in the gap and cause arcing. In general, arcing during fuse operation generates an excessive amount of heat. The excessive amount of heat often results in fracture of the tube enclosing the fuse element. Metallic vapor resulting from the molten fuse element may be ejected from the fuse assembly onto the surrounding circuitry potentially causing a short circuit and potentially resulting in an unsafe operation of the electrical circuit.
Moreover, the printed circuit board area consumed by a fuse may be significant in view of a continued emphasis on miniaturization and increased board densities. A balance of structural strength of the fuse body, the length of the fuse element, and the length of the gap formed by a melting fuse element have been optimized in the TeleLink® SM fuse. Further reductions in size or the required space on a printed circuit board have not been realized due the effect of high-intensity arcing between the two ends of the element. Thus, it is desirable to reduce the footprint of the traditional fuse, while minimizing such arcing.